In a notebook personal computer (hereinafter, referred to as “notebook PC” for simplicity), which is an example of a portable or mobile type electronic device, a lithium ion rechargeable battery is generally used. In recent years, a number of battery pack-related fire accidents has been reported, and safety guidelines have been released for safe use of lithium ion batteries by cooperation of BAJ (The Battery Association of Japan) and JEITA (Japan Electronics and Information Technology Industries Association). The safety guidelines are available from the associations' home pages on the Internet.
According to the safety guidelines, it is recommended that the maximum value(s) of the charging current and/or the charging voltage should be limited depending on the surface temperature of the battery cell when charging the lithium ion batteries.
According to the guidelines (and as shown in FIGS. 9A and 9B) the surface temperature of the battery cell is defined such that a temperature range from T1 to T2 is a low temperature range, a temperature range from T2 to T3 is a standard temperature range, and a temperature range from T3 to T4 is a high temperature range. Although the guidelines state that the values specifying the respective temperature ranges are to be determined by manufacturers through a predetermined test and verification, at present, the values are specified as follows: a lower charging temperature limit T1 is 0 degrees in centigrade (hereinafter similarly), a lower limit of the standard temperature range (an upper limit of the low temperature range) T2 is 10 degrees, an upper limit of the standard temperature range (a lower limit of the high temperature range) T3 is 45 degrees, and an upper charging temperature limit T4 is 55 degrees.
In the guidelines, a maximum charging current and an upper charging voltage limit are specified for each temperature range. The standard temperature range is a surface temperature range of a battery cell in which highest values are applied to the maximum charging current and the upper charging voltage limit. The guidelines state that a standard maximum charging current value Imax1 in the standard temperature range be set to 0.7 ItA and that a standard upper charging voltage limit Vmax1 be set to 4.25 V. At this stage, the unit ItA as used herein represents a unit of current flowing through a secondary battery during charging/discharging and is defined as ItA=(rated battery capacity) (Ah)/1(h). The maximum charging current and the upper charging voltage limit in the low temperature range and the high temperature range are determined by respective manufacturers.
The low temperature range is a temperature range defined to be lower than the standard temperature range and is a surface temperature range of the battery cell during charging, which is permissible on condition that both or either one of the standard maximum charging current value Imax1 and the standard upper charging voltage limit Vmax1 is decreased from the viewpoint of safety. As an example, a low-temperature maximum charging current value Imax2 in the low temperature range is set to 0.3 ItA, and a low-temperature upper charging voltage limit Vmax2 is set to 4.15 V. The high temperature range is a temperature range defined to be higher than the standard temperature range, and is a surface temperature range of the battery cell during charging, which is permissible on condition that both or either one of the standard maximum charging current value Imax1 and the standard upper charging voltage limit Vmax1 is varied from the viewpoint of safety. As an example, a high-temperature maximum charging current value Imax3 in the high temperature range is set to 0.3 ItA, and a high-temperature upper charging voltage limit Vmax3 is set to 4.20 V.
Japanese Laid-open (Kokai) Patent Publication No. 2006-020446 discloses a technology for estimating a battery temperature at a lapse (of X seconds) after the start of charging to control a charging current based on the estimated temperature, thereby preventing the battery temperature from increasing abnormally.
Japanese Laid-open (Kokai) Patent Publication No. 2005-245078 discloses a technology that solves a problem that when an ambient temperature of a secondary battery exceeds an allowable value, charging is stopped and the time to full charging increases. According to the technology described in that patent document, a database is preliminarily constructed by extracting the relationship between the charging current values and an ambient temperature rise in the secondary battery, and when the ambient temperature rises during charging due to environmental influences, the charging current value to the secondary battery is controlled to thereby suppress the temperature rise in the secondary battery, whereby the charging can be completed without interruption.
Japanese Laid-open (Kokai) Patent Publication No. 10-014125 discloses a technology for enabling full charging of a NiMH battery even when charging is started with a condition where a temperature thereof approximately reaches to that near the upper limit of a charge-permissible temperature range. According to the technology described in that patent document, when a present battery temperature has reached the vicinity of the upper limit of the charge-permissible temperature, a current value corresponding to the battery temperature is calculated to reduce the charging current to the calculated current value, thereby ensuring that the battery temperature is prevented from exceeding the upper limit of the battery temperature.
Japanese Laid-open (Kokai) Patent Publication No. 2002-165380 discloses a technology for preventing the temperature of a NiMH battery from reaching a critical level during charging. According to the technology disclosed in that patent document, a charging rate is controlled such that a rate of temperature rise as per unit time (ΔT/t) is kept constant.
The battery cell generates heat during charging and discharging, and the temperature appearing at the surface of a housing of the battery cell (hereinafter, referred to as surface temperature, for the simplicity sake) increases by approximately 7 degrees at the end of the charging. In a battery pack mounted on the notebook PC, when the notebook PC placed in a normal temperature environment is operating while it is supplied with electric power from an AC/DC adapter, the surface temperature becomes about 30 degrees. In such a state, when the AC/DC adapter is detached from the notebook PC and electric power is supplied from the battery pack to the notebook PC, the surface temperature reaches approximately 45 degrees at the end of discharging. Thereafter, when the AC/DC adapter is connected to charge the battery cell, there is a possibility that the surface temperature belongs to the high temperature range before the charging is completed.
When the surface temperature of the battery cell belongs to the high temperature range while charging is performed in the standard temperature range, according to the safety guidelines, it is necessary to lower the standard upper charging voltage limit Vmax1 to the high-temperature upper charging voltage limit Vmax3. Therefore, the charging cannot be performed up to the full charge capacity, with the result that the operation time of the notebook PC during mobile use decreases. Moreover, when the standard maximum charging current value Imax1 is reduced to the high-temperature maximum charging current value Imax3, the time necessary for reaching the full charging is prolonged while bringing such a result that there is a possibility that by the time the AC/DC adapter is removed from the notebook PC for mobile use, the charging has not yet completed to fulfill the condition of the full charge capacity.
Accordingly, a compelling need has been recognized in connection with addressing the above-described shortcomings.